Agreed, but it looks great man, keep it upI have to agree with A Art , the render is over saturated and as a result it doesn't fit with the bg in my opinion. Looks like a cartoon layer over real life. The manipulated background should have also been a lightsource on the render, right now the only lightsource is that crystal, but you have that huge aura in the right background that isn't giving any light. If your going strictly for a photo manip I'd have the render smaller and the image as a whole larger so you could make more alterations. Essentially creating a whole new scene/story. This would allow you to make the render smaller, and help bring it into the enviornment. You could afd a shadow and help bring it into the enviornment by keeping the colors and saturation more consistent.
If you were indeed going for a more signature/photo manip then keep what you have now. Desaturate the render, make it smaller/move it down. Ideally you dont want to cut off the top and sides of a render. In this case you cut off the top. However if the render is already cut off then there is not much you can do. Add some effects in the foreground, right now there is nothing that is bringing it all together. It's essentially a stormtrooper pasted onto a photo manip'd background. If going the sig route i'd say throw in c4ds/fractals and clip masked textures. If going the photo manip route i'd throw in bokehs/unique light sources/fractals and/or shadows. In the case of your background, you can even have elements of a sandstorm layed over your render to help immerse it into the sig.
Kiu, can't wait to see what you make next.